• Assertion (A) is True: The potassium activity ratio ($AR^K = \frac{a_{\text{K}}}{\sqrt{a_{\text{Ca}} + a_{\text{Mg}}}}$) is widely used in soil chemistry instead of absolute K concentration to represent the immediate thermodynamic intensity factor ($I$).
• Reason (R) is True: Plant root membrane transport of potassium is competitively governed by the simultaneous chemical activities of divalent cations ($ ext{Ca}^{2+}$ and $ ext{Mg}^{2+}$) in the soil solution.
• Relationship: Because root potassium absorption depends competitively on calcium and magnesium activity, $AR^K$ provides the physiologically relevant intensity metric, directly explaining Assertion (A).
• Conclusion: Both (A) and (R) are true and (R) is the correct explanation of (A) (Option 1).

Correct option: (1) Both (A) and (R) are true and (R) is the correct explanation of (A)